Sepsis is a systemic inflammatory disorder induced by a dysregulated immune response to infection resulting in dysfunction of multiple critical organs, including the intestines. Previous studies have reported contrasting results regarding the abilities of exosomes circulating in the blood of sepsis mice and patients to either promote or suppress inflammation. Little is known about how the gut epithelial cell-derived exosomes released in the intestinal luminal space during sepsis affect mucosal inflammation. To study this question, we isolated extracellular vesicles (EVs) from intestinal lavage of septic mice. The EVs expressed typical exosomal (CD63 and CD9) and epithelial (EpCAM) markers, which were further increased by sepsis. Moreover, septic-EV injection into inflamed gut induced a significant reduction in the messaging of pro-inflammatory cytokines TNF-α and IL-17A. MicroRNA (miRNA) profiling and reverse transcription and quantitative polymerase chain reaction (RT-qPCR) revealed a sepsis-induced exosomal increase in multiple miRNAs, which putatively target TNF-α and IL-17A. These results imply that intestinal epithelial cell (IEC)-derived luminal EVs carry miRNAs that mitigate pro-inflammatory responses. Taken together, our study proposes a novel mechanism by which IEC EVs released during sepsis transfer regulatory miRNAs to cells, possibly contributing to the amelioration of gut inflammation.

Deregulation of the immune system in sepsis plays the central role in the pathogenesis of multiple organ failure including septic lung injury. Group 2 innate lymphoid cells (ILC2s) have emerged as a new player in regulating immune homeostasis in the lung; however, the role of ILC2s in lung injury in sepsis remains poorly understood. Here, we investigated temporal changes in stimulatory and inhibitory receptor expression and intracellular type 2 cytokine expression of ILC2s in the lung using a cecal ligation and puncture mouse sepsis model. We found that IL-13 production by ILC2s, which were predominately composed of the resident natural ILC2 subset rather than the migratory inflammatory ILC2 subset, was reduced in the lungs of sepsis mice on day 1 and gradually restored through day 7. Although the expression levels of ST2 and inducible T-cell costimulator (stimulatory receptors) were high, IL-13 production by ILC2s was reduced while showing high programmed cell death 1 (PD-1) (inhibitory receptor) expression. Furthermore, using IL-33 knockout mice, we have shown that IL-33 regulates the capacity of ILC2s to produce IL-13, possibly through the modulation of ST2 and PD-1 expression and signaling in the septic lung. To the best of our knowledge, this is the first report showing differential costimulatory/inhibitory receptor expression on ILC2s in a septic lung in the context of an IL-33/IL-13 pathway-mediated type 2 immune response in the progression and resolution of inflammation. Our present findings contribute to a better understanding of the underlying immunological mechanism of ILC2s and may fill the critical knowledge gap regarding immune homeostasis in the lung that hampers the development of new therapeutic strategies for sepsis-induced acute lung injury.

Background: Intensive care unit‐acquired weakness (ICU‐AW) is a syndrome characterized by a long‐term muscle weakness often observed in sepsis‐surviving patients during the chronic phase. Although ICU‐AW is independently associated with increased mortality, effective therapies have yet to be established. Programmed death‐1 (PD‐1) inhibitors have attracted attention as potential treatments for reversing immune exhaustion in sepsis; however, its impact on ICU‐AW remains to be elucidated. Here, we study how PD‐1 deficiency affects sepsis‐induced skeletal muscle dysfunction in a preclinical sepsis model.Methods: Chronic sepsis model was developed by treating wild‐type (WT) and PD‐1 knockout (KO) mice with caecal slurry, followed by resuscitation with antibiotics and saline. Mice were euthanized on days 15–17. Body weights, muscle weights, and limb muscle strengths were measured. Interleukin 13 (IL‐13) and PD‐1 expressions were examined by flow cytometry. Messenger RNA (mRNA) expressions of slow‐twitch muscles were measured by reverse transcription and quantitative polymerase chain reaction (RT‐qPCR). In an in vitro study, C2C12 myotubes were treated with lipopolysaccharide (LPS) and recombinant IL‐13 followed by gene expression measurements.Results: WT septic mice exhibited decreased muscle weight (quadriceps, P < 0.01; gastrocnemius, P < 0.05; and tibialis anterior, P < 0.01) and long‐term muscle weakness (P < 0.0001), whereas PD‐1 KO septic mice did not exhibit any reduction in muscle weights and strengths. Slow‐twitch specific mRNAs, including myoglobin (Mb), troponin I type 1 (Tnni1), and myosin heavy chain 7 (Myh7) were decreased in WT skeletal muscle (Mb, P < 0.0001; Tnni1, P < 0.05; and Myh7, P < 0.05) after sepsis induction, but mRNA expressions of Tnni1 and Myh7 were increased in PD‐1 KO septic mice (Mb, not significant; Tnni1, P < 0.0001; and Myh7, P < 0.05). Treatment of C2C12 myotube cells with LPS decreased the expression of slow‐twitch mRNAs, which was restored by IL‐13 (Mb, P < 0.0001; Tnni1, P < 0.001; and Myh7, P < 0.05). IL‐13 production was significantly higher in ILC2s compared to T cells in skeletal muscle (P < 0.05). IL‐13‐producing ILC2s in skeletal muscle were examined and found to increase in PD‐1 KO septic mice, compared with WT septic mice (P < 0.05). ILC2‐derived IL‐13 was increased by PD‐1 KO septic mice and thought to protect the muscles from experimental ICU‐AW.Conclusions: Long‐term muscle weakness in experimental ICU‐AW was ameliorated in PD‐1 KO mice. ILC2‐derived IL‐13 production in skeletal muscles was increased in PD‐1 KO mice, thereby suggesting that IL‐13 alleviates muscle weakness during sepsis. This study demonstrates the effects of PD‐1 blockade in preserving muscle strength during sepsis through an increase in ILC2‐derived IL‐13 and may be an attractive therapeutic target for sepsis‐induced ICU‐AW.

Sepsis-induced liver injury is common, but the underlying mechanisms remain poorly understood. Given the critical role of gut-liver crosstalk in sepsis, we hypothesize that gut-trained neutrophils, migrating via the portal vein, release neutrophil extracellular traps (NETs) to activate Kupffer cells, thereby exacerbating hepatic injury during sepsis. Here we show that iNOS expression in Kupffer cells increases in septic wild type mice but decreases in PAD4^-/- mice. In vitro, NETs stimulate Kupffer cell IL-6 and TNF release, while conditioned media from NET-treated Kupffer cells induces hepatocyte death. Inhibition of neutrophil elastase and protease-activated receptor-1 (PAR-1) mitigates IL-6 and TNF secretion by Kupffer cells. Ex vivo, portal vein neutrophils from septic mice produce more NETs and induce greater Kupffer cell activation than systemic neutrophils, with this effect attenuated in PAD4^-/- neutrophils. Furthermore, gut intraepithelial lymphocytes (IELs) interact with neutrophils during sepsis and facilitate NETosis, and IEL-primed neutrophils also induce Kupffer cell activation in vitro and in vivo. Our data thus suggest that IEL-facilitated, gut-derived neutrophil NETs activate Kupffer cells to contribute to sepsis-induced liver injury.

Septic shock/severe sepsis is a deregulated host immune system response to infection that leads to life-threatening organ dysfunction. Lung inflammation as a form of acute lung injury (ALI) is often induced in septic shock. Whereas macrophages and neutrophils have been implicated as the principal immune cells regulating lung inflammation, group two innate lymphoid cells (ILC2s) have recently been identified as a new player regulating immune homeostasis. ILC2 is one of the three major ILC subsets (ILC1s, ILC2s, and ILC3s) comprised of newly identified innate immune cells. These cells are characterized by their ability to rapidly produce type 2 cytokines. ILC2s are predominant resident ILCs and, thereby, have the ability to respond to signals from damaged tissues. ILC2s regulate the immune response, and ILC2-derived type 2 cytokines may exert protective roles against sepsis-induced lung injury. This focused review not only provides readers with new insights into the signaling mechanisms by which ILC2s modulate sepsis-induced lung inflammation, but also proposes ILC2 as a novel therapeutic target for sepsis-induced ALI.

Sepsis is a sustained systemic inflammatory condition involving multiple organ failures caused by dysregulated immune response to infections. Sepsis induces substantial changes in energy demands at the cellular level leading to metabolic reprogramming in immune cells and stromal cells. Although sepsis-associated organ dysfunction and mortality have been partly attributed to the initial acute hyperinflammation and immunosuppression precipitated by a dysfunction in innate and adaptive immune responses, the late mortality due to metabolic dysfunction and immune paralysis currently represent the major problem in clinics. It is becoming increasingly recognized that intertissue and/or intercellular metabolic crosstalk via endocrine factors modulates maintenance of homeostasis, and pathological events in sepsis and other inflammatory diseases. Exosomes have emerged as a novel means of intercellular communication in the regulation of cellular metabolism, owing to their capacity to transfer bioactive payloads such as proteins, lipids, and nucleic acids to their target cells. Recent evidence demonstrates transfer of intact metabolic intermediates from cancer-associated fibroblasts via exosomes to modify metabolic signaling in recipient cells and promote cancer progression. Here, we review the metabolic regulation of endothelial cells and immune cells in sepsis and highlight the role of exosomes as mediators of cellular metabolic signaling in sepsis.